In agricultural forage harvesters the sharpness of the chopper knives is a significant factor in the quality of the cut and is also a factor in the spacing between the shear bar and the chopper knives, and the power required for the cutting operation, since the cutting force increases significantly with dull chopper knives. Usually the operator of the forage harvester recognizes from the cutter sounds, the rotational speed of the drive motor and/or the quality of the chopper cut the time at which a grinding process is required, in order to sharpen the chopper knives again. Here it is seen as a disadvantage that the recognition of the sharpness of the chopper knives by the operator is subject to subjective influences and determinations and accordingly is not very exact. Since the sharpness of the chopper knives at the beginning of the grinding cycle is not known precisely, the determination of the duration of the grinding is also problematic, so that in many cases either too much or too little material is ground off the chopper knives, which leads in the first case to unnecessary wear, and in the second case to inadequate sharpness of the chopper knives.
In order to improve the accuracy of the recognition of the sharpness of the chopper knives and to control the grinding process automatically, various procedures have been described.
DE19903153C1 proposes measuring the forces applied by the chopper knives to the shear bar in the radial, as well as the tangential direction, and to form a quotient from these that represents a measure of the sharpness of the chopper knives.
According to DE10235919A1 the acceleration of the shear bar is detected and subjected to a frequency analysis. On the basis of the harmonic wave spectrum a determination can be made whether the chopper knives are still sufficiently sharp or not.
Moreover, DE4133043A1 proposes that the number of cutting operations in a cutting machine be detected in order to initiate a grinding process and to initiate a grinding process after reaching a predetermined number of cutting operations.
Finally US 2007/0209344 A1 describes a lawn mower in which the drive power of the cutting spindle is detected. If it exceeds a predetermined threshold value, the operator is advised to perform a grinding operation.
In the state of the art it is seen as detrimental that the procedure based on the detection of the cutting angle, according to DE19903153C1 and DE10235919A1, does not always operate accurately enough, since the cutting forces depend not only on the sharpness of the chopper knives and their distance from the shear bar, but also upon mechanical properties of the chopped harvested crop and its through put. A direct determination of the number of cutting processes according to DE4133043A1 is not feasible in the case of a forage harvester, while a detection of the drive power requirement analogous to US 2007/0209344 A1 would also not lead to sufficiently accurate measurement values due to the effect of the material properties of the harvested crop and the spacing between the shear bar and the chopper knives.